public interface ThreeD {
  /**
   * Get the depth of this <tt>ThreeD</tt> object.
   * @return the depth of this <tt>ThreeD</tt> object
   */
   double getDepth();

   /**
    * Set the depth of this <tt>ThreeD</tt> object.
    * If the argument <tt>theDepth</tt> is <= 0.0, it is ignored and the
    * <tt>ThreeD</tt> object is unchanged.
    * @param theDepth the new depth of this <tt>ThreeD</tt> object; must be > 0.0
    */
   void setDepth( double theDepth );

   /**
    * Get the volume of this <tt>ThreeD</tt> object.
    * @return the volume of this <tt>ThreeD</tt> object
    */
   double getVolume();
}